In many sectors of the national infrastructure, but most especially in critical sectors such as nuclear power, alarm systems which detect the breaching of a perimeter fence, wall or other barrier must be promptly and reliably detected.
Many such alarm systems are known in the prior art but most suffer from one or more of the following problems:
(1) they rely on vibration sensors tuned specifically to detect cutting or climbing of fence fabric, and are thus unreliable to detect the opening of gates or doors in the fence;
(2) they rely on additional sensors (e.g., position switches, infrared beams, microwave beams, etc.);
(3) the opening of a portal within the barrier does not create a vibration sufficiently strong to reliably trigger a vibration detector;
(4) the vibration is set off by a breach of sufficient magnitude to threaten the vibration sensors with damage;
(5) the devices are not passive in nature and cannot create detectable disturbances from the energy of a portal breach alone;
(6) the devices increase the force required to open the portal barrier beyond the limits of applicable building codes;
(7) the devices are unreliable when the portal barrier is opened very slowly or very rapidly;
(8) the devices are not self-resetting on closure of the portal barrier;
(9) the devices actually prevent the reclosure of an otherwise self-closing portal barrier;
(10) the devices rely solely upon a single disturbance which may or may not be detectable;
(11) the devices rely on detectable disturbances which do not necessarily occur within the first six inches or so of portal barrier motion;
(12) the devices rely on vibratable elements having a bending stress greater than the yield stress of one or more elements, therefore risking damage to the device;
(13) the devices are unduly large; and
(14) the devices are susceptible to tampering.
Accordingly, there is a need for a portal entry security device which avoids the aforementioned problems in the prior art.